The last will and testament of the Piast duke Bolesław III Wrymouth of Poland,[1] established rules for governance of the Polish kingdom by his four surviving sons after his death. By issuing it, Bolesław planned to guarantee that his heirs would not fight among themselves, and would preserve the unity of his lands under the House of Piast. However, he failed; soon after his death his sons fought each other, and Poland entered a period of fragmentation lasting about 200 years.[2]

The youngest son Casimir II the Just was not assigned any province; it is speculated that he was born after Bolesław's death, or he was destined for a religious career.

The senioral principle established in the testament stated that at all times the eldest member of the dynasty (the Senior Prince, the Princeps or High Duke) was to have supreme power over the rest (Dux, the Dukes) and was also to control an indivisible "seniorate province" : a vast strip of land running north-south down the middle of Poland, with Kraków (the Kingdom of Poland's capital) its chief city. The Senior's prerogatives also included control over the Pomeranian vassals in Pomerelia, a fief. The Senior was tasked with defense of borders, the right to have troops in provinces of other Dukes, carrying out foreign policy, supervision over the clergy (including the right to nominate bishops and archbishops), and minting of currency.

The senioral principle was soon broken, with Władysław II attempting to increase his power and his younger half-brothers opposing him. After initial success (taking over the Łęczyca Land after the death of Salomea), he was eventually defeated and expelled from Poland in 1146. Only with the help of Emperor Frederick I Barbarossa his sons managed to retain the Silesian Province in 1163, losing the Seniorate, which had passed to their uncle Bolesław IV. This led to a period of nearly 200 years of Poland's feudal fragmentation; the estrangement of the Silesian Piasts deepening after the death of Duke Henry II the Pious at the disastrous Battle of Legnica in 1241.